1. Field of the Invention
The present invention relates to a holographic storage medium, and a method and an apparatus for recording/reproducing data on/from the holographic storage medium.
2. Description of the Related Art
In optical holography, data is stored in the volume, not the surface, of a recording medium. Holograms are recorded using a signal beam and a reference beam. The signal beam interferes with the reference beam in the recording medium, to generate an interference pattern, which is referred to as a datapage. Multiple patterns can be superimposed, by changing the optical characteristics of the reference beam. This process is referred to as “multiplexing.” When reading data, a single reference beam is incident on the recording medium, under the same conditions as in data recording, to generate a diffraction beam, which displays stored data in a page (datapage). The diffraction beam is detected by a detection array. The detection array extracts stored data bits, by measuring intensity patterns. The datapage includes a large number of data bits or pixels. Data storage capacity can be increased, by superimposing additional datapages in the same volume of the recording medium.
FIGS. 1A and 1B illustrate the recording and reproducing of data in optical holography. Referring to FIG. 1A, when recording data, a reference beam R and a signal beam S interfere with each other, to generate an interference pattern in a storage medium. Referring to FIG. 1B, when reproducing data, the original reference beam R is applied to holograms stored in the storage medium, to generate the output signal beam S, by diffracting the recorded hologram.
Data is recorded on the holographic storage medium by an interference between the signal beam S and the reference beam R. The signal beam S is generated by a spatial light modulator (SLM). The signal beam S is in the form of a datapage that includes a plurality of pixels. The signal beam S passes through an optical system, and interferes with the reference beam R, to produce an interference pattern on the storage medium. The interference pattern, generated by an interference of the signal beam S and the reference beam R, is recorded in the storage medium. If the reference beam R is applied to the recorded interference pattern, the recorded signal beam S is reproduced by diffraction.
When recording the holograms, the intensity and phase of the signal beam S can be recorded by, for example, varying the angle of the reference beam R. Hundreds or even thousands of hologram datapages can be recorded in the same position. The datapage contains information in the form of pixels, in particular, when the pixels are turned on or off. The original data is modulated, by adjusting the number of on or off pixels in one binary datapage, while recording the data on the holographic storage medium.
FIGS. 2A-2C show examples of conventional modulation codes, that are used for producing holograms. The modulation code lowers the ratio of light to dark portions, by distributing light evenly, to reduce errors. In the modulation code, for example, “1” and “0” can be respectively expressed as “01” and “10”. Alternatively, “1” and “0” can be respectively expressed as “10” and “01”. Assuming that “1” is a case when light is allowed to pass through a pixel, and “0” is a case when light is blocked, if the data includes many 0's, power is not sufficiently supplied to the holograms, and recording is not performed efficiently. If the data includes too many 1's, a signal produced therefrom may be degraded. The modulation code is used to prevent this problem and to improve signal quality. Data before modulation is referred to as a dataword. A dataword is referred to as a codeword, after being modulated by a modulation code.
Referring to FIGS. 2A through 2C, white portions are on-pixels and shaded portions are off-pixels. The on-pixels transmit incident rays and the off-pixels block incident rays. FIG. 2A shows a conventional 1:2 modulation code, which is a 2-bit modulation code for representing 1-bit data. In the 1:2 modulation code, 1-bit information of “0” or “1” can be indicated in accordance with the position of an on-pixel. One (1) on-pixel and one (1) off-pixel are included in the 1:2 modulation code, allowing two (2) permutations. Since 21=2, 1-bit data can be represented by this modulation code. Therefore, the modulation code of FIG. 2A is a 1:2 modulation code.
FIG. 2B shows a conventional 2:4 modulation code, which can represent four (4C1) different 2-bit datawords, i.e., “00”, “01”, “10”, and “11”. Since 22=4, a 4-bit modulation code is needed to represent 2-bit data. Therefore, the modulation code of FIG. 2B is a 2:4 modulation code.
FIG. 2C is a view of a conventional 3:9 modulation code. 3-bit datawords are modulated to 9-bit codewords. One pixel out of nine pixels is the on-pixel, giving nine (9) permutations. Since 9 is a number between 23 and 24, all 3-bit datawords can be represented, but not all 4-bit datawords can be represented. Accordingly, 23=8 different datawords can be represented by 3-bit codewords, using the 3:9 modulation code.
When a lens is used to record and reproduce data in holography, different parts of the lens have different field characteristics. In general, field characteristics, such as the modulation transfer function (MTF), are high in the center of the lens and lower towards the edges of the lens. The MTF is a variation of the spatial frequency characteristics of a signal and is an indication of the characteristics and performance of a lens. When field characteristics, such as the MTF, deteriorate, the signal quality of a datapage is reduced. For example, the signal quality can be reduced in signals traveling through the edges of a lens. Furthermore, in the modulation code used for the holograms, the quality of a reproduction signal, such as bit error rate (BER), is improved as the number of on-pixels is decreased. However, if the on-pixel rate is fixed, the signal characteristics of the datapage data vary in accordance with the position of the lens. Therefore, signals of consistent quality cannot be recorded or reproduced.